This application is based on and incorporates herein by reference Japanese Patent Application No. 2002-174553 filed on Jun. 14, 2002.
1. Field of the Invention
The present invention relates to an exhaust gas cleaning system for an internal combustion engine, in particular, to a method of regenerating a particulate filter.
2. Description of Related Art
Lately, improvement in exhaust emission of an internal combustion engine mounted on a vehicle and the like is required. Specifically, exhaust particulate matters such as soot or a soluble organic fraction included in the exhaust gas discharged from a compression-ignition type diesel engine running on diesel oil should be removed, as well as carbon oxide, hydrocarbon and nitrogen oxide. Therefore, a particulate filter made of a porous material is disposed in an exhaust passage of the diesel engine for collecting the exhaust particulate matters included in the exhaust gas.
When the exhaust gas passes through porous filter walls of the particulate filter, the particulate matters included in the exhaust gas are filtered at surfaces or pores of the filter walls. If an excessive quantity of the particulate matters is collected, flow resistance at the particulate filter may increase. As a result, back pressure of the engine may be increased, and the engine output may be decreased. Therefore, ability of the particulate filter to collect the exhaust particulate matters should be recovered by regenerating the particulate filter. The particulate filter is regenerated by eliminating the collected exhaust particulate matters.
A particulate filter having an oxidation catalyst such as platinum thereon can be regenerated during the operation of the engine with the use of an oxidization effect of the oxidization catalyst. For instance, a post-injection for injecting fuel in an expansion stroke of the engine is performed at a predetermined timing in order to provide the fuel to the particulate filter. The temperature of the oxidization catalyst is increased with the use of heat, which is generated by combusting the fuel. Thus, the collected particulate matters are eliminated. Otherwise, the timing of the normal fuel injection is retarded to decrease efficiency of the engine. Thus, waste heat, which is not converted into motive energy, is increased, and the temperature of the oxidization catalyst is increased with the use of the waste heat. Thus, the particulate matters depositing in the particulate filter are combusted and eliminated.
In a method disclosed in Japanese Patent Unexamined Publication No. H11-13455 (first example), a quantity of exhaust particulate matters generated in an internal combustion engine main body is calculated based on measured engine rotation speed and a flow rate of fuel. Then, a quantity of the exhaust particulate matters collected by the particulate filter is estimated by integrating the quantity of the particulate matters generated in the engine. In this method, a map relating the engine rotation speed and the fuel flow rate with the quantity of the generated exhaust particulate matters is used. Data in the map are obtained by calculating a generating quantity of the exhaust particulate matters based on various rotation speeds and flow rates of the fuel, through benchmark tests and the like.
However, in order to measure the quantity of the collected particulate matters precisely, accuracy of the map has to be improved by minutely segmenting the engine rotation speed and the flow rate of the fuel. Since the data of the map are obtained in a steady operating state, an error will be generated if the map is used in a transitional state. The transitional state is formed many times until the quantity of the collected particulate matters increases up to a level at which regeneration of the particulate filter is required. Therefore, the errors are accumulated, and timing of the regeneration may become erroneous.
In another method disclosed in Japanese Patent Unexamined Publication No. H07-332065 (second example), timing to start the regeneration of the particulate filter is determined based on a pressure difference between an inlet and an outlet of the particulate filter. The flow resistance at the particulate filter increases as the quantity of the collected particulate matters increases. The pressure difference increases as the flow resistance increases. Therefore, it is determined that the regeneration should be started at the timing when the pressure difference exceeds a predetermined value.
However, the pressure difference is small if the flow rate of the exhaust gas passing through the particulate filter is small. Therefore, the collection quantity of the particulate matters cannot be necessarily measured with adequate accuracy. Moreover, a steady pressure difference cannot be obtained in the transitional state. As a result, the measuring accuracy may be deteriorated.
It is therefore an object of the present invention to provide an exhaust gas cleaning system for an internal combustion engine capable of determining timing for regenerating a particulate filter appropriately.
According to an aspect of the present invention, an internal combustion engine has a particulate filter, which is disposed in an exhaust pipe for collecting particulate matters included in the exhaust gas discharged from cylinders of an engine main body and is regenerated at a predetermined timing by eliminating the collected particulate matters. An exhaust gas cleaning system of the engine has passing state detecting means, operating state detecting means, measuring accuracy determination value calculating means, measuring accuracy determining means, first collection quantity calculating means, collection quantity increment value calculating means, second collection quantity calculating means, regeneration determining means and regeneration performing means. The passing state detecting means detects a passing state of the exhaust gas through the particulate filter. The operating state detecting means detects an operating state of the engine main body. The measuring accuracy determination value calculating means calculates a measuring accuracy determination value based on the detected operating state or the detected passing state. The measuring accuracy determining means determines whether the measuring accuracy of a collection quantity of the collected particulate matters is higher than a threshold measuring accuracy or not by comparing the measuring accuracy determination value with a predetermined value. The first collection quantity calculating means calculates the collection quantity based on the detected passing state of the exhaust gas when the measuring accuracy is determined to be higher than the threshold measuring accuracy. The collection quantity increment value calculating means calculates a discharge quantity per unit time of the particulate matters discharged from the engine main body based on the detected operating state of the engine main body. The collection quantity increment value calculating means also calculates a collection quantity increment value per unit time from the calculated discharge quantity per unit time when the measuring accuracy is determined to be lower than the threshold measuring accuracy. The second collection quantity calculating means calculates the collection quantity by adding the collection quantity increment value to the previous collection quantity when the measuring accuracy is determined to be lower than the threshold measuring accuracy. The regeneration determining means determines whether the collection quantity, which is calculated by the first or second collection quantity calculating means, is greater than a threshold collection quantity or not. The regeneration performing means performs the regeneration of the particulate filter if the collection quantity is determined to be greater than the threshold collection quantity.
When the engine is in a steady operating state and the measuring accuracy is high, the collection quantity of the particulate matters is calculated based on the passing state of the exhaust gas at the particulate filter. If the operating state becomes transitional and the measuring accuracy decreases, the collection quantity at that time is calculated by accumulating the collection quantity increment values with a base portion. The base portion is the collection quantity calculated based on the passing state of the exhaust gas while the measuring accuracy is high. The collection quantity increment value is calculated based on a quantity of the discharged particulate matters, which is estimated from the operating state of the engine main body. Therefore, a most part of the measurement error of the collection quantity is included in the accumulated collection quantity increment values, which are calculated after the last calculation of the collection quantity based on the passing state of the exhaust gas. Therefore, the entire errors of the collection quantity increment values since the system was used first time do not accumulate unlike the first example of the related art. In the first example, the entire errors of the collection quantity increment values will accumulate because the present collection quantity is calculated by accumulating the collection quantity increment values since the first use of the system (or since the last regeneration of the system).
The base portion of the collection quantity is the collection quantity previously calculated based on the passing state of the exhaust gas at the particulate filter when the engine is in the steady operating state and the measuring accuracy is high. Therefore, generation of a great error can be prevented regardless of the operating condition of the engine main body, unlike the second example of the related art. In the second example, a great error may be generated in some operating conditions because the present collection quantity is calculated based on the passing state of the exhaust gas at the particulate filter at the time when the present collection quantity is calculated.